Flash memory is non-volatile, which means that it stores information on a semiconductor in a way that does not need power to maintain the information in the chip. Flash memory is based on the Floating-Gate Avalanche-Injection Metal Oxide Semiconductor (FAMOS transistor), which is essentially a Complimentary Metal Oxide Semiconductor (CMOS) Field Effect Transistor (FET) with an additional conductor suspended between the gate and source/drain terminals. Current flash memory devices are made in two forms: NOR flash and NAND flash. The names refer to the type of logic used in the storage cell array. Further, flash memory stores information in an array of transistors, called “cells”, each of which traditionally stores one or more bits of information.
A flash cell is similar to a standard MOSFET transistor, except that it has two gates instead of just one. One gate is the control gate like in other MOS transistors, but the second is a floating gate that is insulated all around by an oxide layer. The floating gate is between the control gate and the substrate. Because the field gate is isolated by its insulating oxide layer, any electrons placed on it get trapped there and thus store the information.
When electrons are trapped on the field gate, they modify (partially cancel out) an electric field coming from the control gate, which modifies the threshold voltage (Vt) of the cell. Thus, when the cell is “read” by placing a specific voltage on the control gate, electrical current will either flow or not flow between the cell's source and drain connections, depending on the Vt of the cell. This presence or absence of current is sensed and translated into 1's and 0's, reproducing the stored data.
One application of non volatile memory is portable devices. Conservation of electrical power is desired in portable devices. Moreover, non volatile memory can be slower to operate than volatile memory. It is desired to speed operation of flash memory.